In the wikipedia page of Zermelo-Fraenkel set theory, the first axiom (Axiom of extensionality) says that two sets are equal (are the same set) if they have the same elements.
Now it is said that the converse of this axiom follows from the substitution property of equality which is "For any quantities $a$ and $b$ and any expression $F(x)$, if $a = b$, then $F(a) = F(b)$".
I don't understand the argument here on substitution.